In wireless mobile communications, a channel that couples a transmitter to a receiver is often time-varying due to relative transmitter-receiver motion and multipath propagation. Such a time-variation is commonly referred to as fading, and may severely impair system performance. When a data rate for the system is high in relation to channel bandwidth, multipath propagation may become frequency-selective and cause intersymbol interference (ISI). By implementing Inverse Fast Fourier Transform (IFFT) at the transmitter and FFT at the receiver, Orthogonal Frequency Division Multiplexing (OFDM) converts an ISI channel into a set of parallel ISI-free subchannels with gains equal to the channel's frequency response values on the FFT grid. Each subchannel can be easily equalized by a single-tap equalizer using scalar division.
To avoid inter-block interference (IBI) between successive IFFT processed blocks, a cyclic prefix (CP) of length greater than or equal to the channel order is inserted per block at the transmitter and discarded at the receiver. In addition to suppressing IBI, the CP also converts linear convolution into cyclic convolution and thus facilitates diagonalization of an associated channel matrix.
Instead of having multipath diversity in the form of (superimposed) delayed and scaled replicas of the transmitted symbols as in the case of serial transmission, OFDM transfers the multipath diversity to the frequency domain in the form of (usually correlated) fading frequency response. Each OFDM subchannel has its gain being expressed as a linear combination of the dispersive channel taps. When the channel has nulls (deep fades) close to or on the FFT grid, reliable detection of the symbols carried by these faded subcarriers becomes difficult if not impossible.
Error-control codes are usually invoked before the IFFT processing to deal with the frequency-selective fading. These include convolutional codes, Trellis Coded Modulation (TCM) or coset codes, Turbo-codes, and block codes (e.g., Reed-Solomon or BCH). Such coded OFDM schemes often incur high complexity and/or large decoding delay. Some of these schemes also require Channel State Information (CSI) at the transmitter, which may be unrealistic or too costly to acquire in wireless applications where the channel is rapidly changing. Another approach to guaranteeing symbol detectability over ISI channels is to modify the OFDM setup: instead of introducing the CP, each IFFT-processed block can be zero padded (ZP) by at least as many zeros as the channel order.